1. Field of the Invention
This invention is directed to a transfer printing process and article wherein image-carrying ink is sandwiched between elastomeric thermally setting synthetic resin ink, the bottom layer of which is adhesively applicable to a stretchable fabric.
2. Description of the Prior Art
As discussed below, the usual printing methods are not well suited to directly applying an image onto a fabric, and particularly a stretchable fabric. The two basic printing methods used to transfer an ink to organize a permanent image are: surface-to-surface transfer of the ink, and application through a stencil. Several photographic or electrostatic printing processes are also known, but are not economically feasible to apply an image to a stretchable fabric in mass quantities.
Surface-to-surface printing methods include letterpress, rotary press, intaglio or gravure, web-fed offset and sheet-fed offset. Hectograph and ditto processes also fall under this category, but will not be discussed for image application to a fabric.
Letterpress utilizes a raised surface impression and is limited to single sheet feed. The material being printed must have reasonable linear rigidity to allow proper feed, whereas textiles do not normally have this linear rigidity. Rotary press utilizes raised surface impression as with letterpress, but has the advantage of high speed continuous roll feed. The material being printed must also have reasonable linear rigidity, and must have a fairly smooth and even surface not normally found with textiles. By substituting a rubber printing plate for the normal metal plates of the rotary or letter press, somewhat irregular surfaces such as corrugated cardboard can be printed. However, due to the elasticity of the rubber plate, fine half-tone screens or close color registration is impractical.
Web-fed and sheet fed offset processes are a high speed adaption of the lithographic process and utilize the principal that oil or grease based inks will not mix with water. By photo-chemically treating a thin metal or paper sheet, certain positions of the plate form an affinity with water, preventing an ink deposit, while other portions are left free to transfer ink. This process does not require a raised impression and proves most economical for full color or monochromatic fine half-tone printing. Again, the offset process requires linear rigidity, and in particular, a smooth surfaced material for printing. Gravure utilizes a recessed, intaglio, image surface.
The most important factor in the surface-to-surface ink transfer process is that all surfaces being printed must come in direct pressure contact with the printing plate surface. Also, the maximum deposit of ink possible in these processes is measured in a few (1 to 5) microns thickness. Gravure methods can deposit heavier layers of ink, and have been used by some textile printers to print fabrics. However, the ink transfered cannot be controlled closely enough to allow full color or fine monochrome halftones. The high cost of gravure printing plates has caused many textile printers to abandon this method for textile printing.
The stencil printing method has the advantage of depositing heavy layers of ink on smooth or rough surfaces at low stencil costs. The rigidity or flexibility of the material being printed is unimportant, as it normally does not move during the printing process. Common stencil printing is achieved by the open mask stencil, or by silk-screen, serigraphic, process. The mimeograph process also falls in this category, but is impractical for textile type applications.
Of the stencil printing processes, silk-screen printing is the most versatile for textile or similar applications. This process employs a fine mesh silk or other synthetic fiber stretched tightly over a rigid frame. A mask of gelatin, varnish or similar material is adhered to the silk screen, with the print pattern cut out to allow passage of ink to the material being printed.
The silk-screen material is available in several meshes to accommodate various needs. The finer the screen mesh, the finer the detail which can be achieved. Fine mesh screens require thinner ink viscosity and consequently deposit thinner layers of ink. Coarse mesh screens will allow heavy deposits of more viscous inks, but do not allow fine detail.
By photosensitizing the gelatin or other masking material, a halftone reproduction can be achieved. However, the halftone screen mesh and the actual silk-screen mesh are conflicting, and fine definition of the halftone is not possible. It is generally considered by silk-screen printers that under optimum conditions and skills, the maximum halftone screen practical is 65 to 80 line. This requires the use of the finest silk-screen mesh available and exceptional skills by the screenprint operator. Thus, very fine monochromatic halftones are impractical. In addition, in multicolor work precise registration is necessary for full color printing. Furthermore, precise deposits of ink are necessary for full color printing to make full color silk-screen printing impractical.
The silk-screen process has been chosen by most in the textile printing industry to enable a heavy deposit of ink which will be absorbed deep into the fibers, creating a solid color image, at low screen-print costs. Multiple solid color printing with reasonable accuracy in registration is accomplished by placing the fabric on a stationary surface, and then applying several different solid colors with the use of as many different silk screens. During this process, the fabric is not normally moved or repositioned on the stationary printing surface.
With regard to inks, the most recent advances in silk-screen process printing of fabrics has been the advent of thermal stretch ink which is thermally setting plastisol. In these inks, vinyl polymer is dispersed in plasticiser and optional polar solvent. Pigments can be mixed in the polymer and plasticiser and are thermally set by the application of heat to form a thermoplastic ink which remains resilient, but has a sufficiently high softening temperature that it can normally withstand maximum wash temperatures without changes in the ink characteristics.